The respiratory action of the intercostal muscles will be analyzed. The principles of engineering mechanics yield the relation between muscle force and the change in pleural pressure that the force produces. Data that describe the geometry and motion of the parasternal region of the dog will be obtained to augment existing data on the geometry and motion of dog and human ribs. The distributions of intercostal muscle orientation and mass in dogs and humans will be measured. From these data, the factors that appear in the relation between force and pleural pressure will be evaluated and the potential respiratory action of the intercostal muscle will be calculated. The relation between force and pleural pressure will be demonstrated in a controlled experiment in which known forces will be applied to the rib cage to simulate the action of the intercostals, and limits of the theory imposed by geometrical and mechanical nonlinearities will be determined. A quantitative description of the respiratory mechanics of the intercostal muscles will be obtained.